Heat exchangers made of resin material are superior in corrosion resistance and heat resistance but inferior in mechanical strength to metallic heat exchangers. Among resin materials, fluorocarbon resin is particularly excellent in corrosion resistance and heat resistance, but the mechanical strength thereof is relatively low. This means that a heat exchanger made of fluorocarbon resin cannot be used unless the supply fluid pressure is lowered below a predetermined level, which is one of the causes of narrowing the range of use of fluorocarbon-resin heat exchangers. To supply industrial circulating water into a heat exchanger, for example, a return pressure higher than a predetermined level needs to be ensured for the circulating water; therefore, the supply fluid pressure cannot be reduced considerably. For this reason, fluorocarbon-resin heat exchangers cannot be used unless some measures are taken to improve pressure resistance.
To improve the pressure resistance of a heat exchanger made of fluorocarbon resin, it is particularly important to improve the pressure resistance of the joint at the distal end of the heat exchanger. A flange joint structure and a threaded joint structure are generally known as examples of heat exchanger distal end joint structures capable of improving pressure resistance. The flange joint structure is capable of improving pressure resistance relatively easily but results unavoidably in an increased size of the joint. The threaded joint structure, on the other hand, allows the joint to become relatively small in size but frequently includes a packing or O-ring installed in the joint. In this case, if the packing or O-ring should become deteriorated, the fluid flowing through the heat exchanger may leak. The deterioration of the packing or O-ring disposed in the joint cannot be confirmed from the outside. Therefore, it is necessary, in order to surely avoid fluid leakage, to replace the packing or O-ring very frequently, or to perform inspection by disassembling the joint. Thus, the threaded joint structure is inferior in maintainability. Basically, an effective way to improve the mechanical strength of the threaded joint structure is to increase the wall thickness of the coupler, which, however, causes an increase in size of the coupler and hence leads to an increase in cost.